1. Field of the Invention
This invention relates to an air-fuel ratio control apparatus for an internal combustion engine, for example, a V-type multi-cylinder engine composed of two cylinder groups, and more particularly to an air-fuel ratio control apparatus for an internal combustion engine of a type arranged in such a manner that air-fuel ratios of cylinder groups respectively are controlled to different phases to prevent change in rotations and improve an effect of a trinary catalyst.
2. Description of the Related Art
Hitherto, an air-fuel ratio control apparatus of the foregoing type has been arranged as disclosed in Japanese Patent Publication No. 60-53771 in such a manner that a integral output means controls the air-fuel ratio of a first cylinder group in accordance with a signal transmitted from an air-fuel ratio sensor disposed in only an exhaust pipe of a first cylinder group, and a rectangular wave signal having an inverse phase to that of the integral output means is used to control a second cylinder group. As a result, the concentrations of the air-fuel ratios of the two cylinder groups are made different.
FIG. 9 is a structural view which illustrates an air-fuel ratio control apparatus for an internal combustion engine adapted to a V-type and 8-cylinder engine disclosed as described above. Referring to FIG. 9, reference numeral 1 represents an engine body composed of a first cylinder group 1a and a second cylinder group 1b.
An ECU (an electronic fuel injection control unit) 8A comprises a main calculating circuit 81A, a correction circuit 87a for making fuel injection signal C1 to be supplied to the first cylinder group 1a and a correction circuit 87b for making fuel injection signal C2 to be supplied to the second cylinder group 1b. The correction circuits 87a and 87b also have functions of operating injectors (omitted from illustration) of the corresponding cylinder groups 1a and 1b.
An air-fuel ratio sensor 15 is disposed in an exhaust pipe 14a of the first cylinder group 1a to detect the air-fuel ratio of a mixed gas in the exhaust pipe 14a. Trinary catalysts 16a and 16b are disposed downstream from the corresponding exhaust pipes 14a and 14b to purify the exhaust gas in each of the exhaust pipes 14a and 14b. Reference numeral 25 represents a known feedback control circuit for comparing and integrating air-fuel ratio signal AF supplied from the air-fuel ratio sensor 15. Reference numeral 26 represents a correction control circuit for generating inverse phase signal B2 to be supplied to the second cylinder group 1b in response to an output signal transmitted from the feedback control circuit 25.
The feedback control circuit 25 includes a comparison circuit and an integrating circuit, the feedback control circuit 25 transmitting output signal B1 to be received by the correction circuit 87a disposed in the ECU 8A. The inverse phase signal B2 is, by way of the correction control circuit 26, received by the correction circuit 87b.
Referring to a waveform graph shown in FIG. 10, the operation of the conventional air-fuel ratio control apparatus for an internal combustion engine shown in FIG. 9 will now be described. It should be noted that the ECU 8A receives signals transmitted from various sensors (omitted from illustration) which detect various states of the operation.
First, the main calculating circuit 81A disposed in the ECU 8A calculates the basic quantity of fuel to be injected per unit rotation of the engine in accordance with an air suction quantity or the like detected by an air flow sensor (omitted from illustration).
Then, the correction circuits 87a and 87b correct the basic air injection quantity in accordance with the temperature of water for cooling the engine detected by a temperature sensor (omitted from illustration) and so forth to supply information about the corrected quantity to the injectors of the cylinder groups 1a and 1b as fuel injection signals C1 and C2.
The first cylinder group 1a is, at this time, feedback-controlled in response to the air-fuel ratio signal AF so that the air-fuel ratio in the exhaust pipe 14a is adjusted to satisfy a theoretical air-fuel ratio (14.7). The air-fuel ratio of the second cylinder group 1b is open-loop-controlled as to satisfy the theoretical air-fuel ratio in such a manner that it is increased and/or decreased in an inverse phase with respect to the air-fuel ratio of the first cylinder group 1a.
That is, the correction circuit 87a performs calculations for the correction in response to the output signal B1 transmitted from the feedback control circuit 25, while the correction circuit 87b performs calculations for the correction in response to the inverse phase signal B2 supplied by way of the correction control circuit 26. The correction control circuit 26 superposes the average of rectangular wave output signals and that of integral output signals in inverse phase, the levels of which are lowered when the integral output signals transmitted from the feedback control circuit 25 are increased and which are raised when the same are decreased. The correction control circuit 26 generates an inverse phase signal B2 to be supplied to the correction circuit 87b.
Therefore, the fuel injection signals C1 and C2 are formed into waveforms that increase and decrease in mutually inverse phases as shown in FIG. 10.
Since the alternate supply of the thick and thin air-fuel ratio air to each of the cylinder groups 1a and 1b realizes the average theoretical air-fuel ratio in the trinary catalysts 16a and 16b, an efficiency of purifying the exhaust gas can be improved. That is, HC and CO generated in a rich control mode can be, in an average manner, mixed with NOx generated in a lean control mode. Since factors, which vary the engine revolutions between the two cylinder groups 1a and 1b, can be offset, the change in the engine revolutions can be prevented.
However, if specifications or operation conditions are different such that sucked air is irregularly distributed due to machining deviations between the two cylinder groups 1a and 1b or due to the structure and layout of the suction pipes or such that the temperature of the sucked air or the engine is different due to the structure and the layout of the cooling water passage and the exhaust pipes 14a and 14b, the structure made such that the air-fuel ratio in the second cylinder group 1b is open-loop-controlled results in that the air-fuel ratio of the second cylinder group 1b is not always controlled to a predetermined theoretical air-fuel ratio. Therefore, there arises a risk that the improvement in the efficiency of purifying exhaust gas and prevention of the change in the revolutions of the engine cannot be realized.
If the engine speed is accelerated or decelerated, a state is sometimes continued in which both of the air-fuel ratios of the two cylinder groups 1a and 1b are rich (or lean).
An example state will now be considered in which both of the air-fuel ratio of the cylinder group 1a and that of the cylinder group 1b are rich. Since the first cylinder group 1a is feedback-controlled at this time in response to the air-fuel ratio signal AF supplied from the air-fuel ratio sensor 15, the control is so performed that the air-fuel ratio of the first cylinder group 1a is made lean in order to approximate the air-fuel ratio to the theoretical air-fuel ratio.
On the contrary, the air-fuel ratio of the second cylinder group 1b is controlled in an inverse direction to the direction in which that of the first cylinder group 1a is controlled. It leads to a fact that the air-fuel ratio of the second cylinder group 1b is controlled to the rich side though the actual air-fuel ratio is rich.
Also a state can be realized such that the air-fuel ratio of the second cylinder group 1b is made lean though a lean air-fuel ratio has been realized.
FIG. 11 is a waveform graph which shows changes of the fuel injection signals C1 and C2 taken place at the time of the acceleration of the engine, wherein the acceleration has taken place at time t0.
Assuming that the state where both of the air-fuel ratios of the respective cylinder groups 1a and 1b have been continued in the foregoing case, the air-fuel ratio of the first cylinder group 1a is approximated to the theoretical air-fuel ratio by continuously increasing, toward the rich (thick) side, the fuel injection signal C1 to be supplied to the first cylinder group 1a. On the other hand, the fuel injection signal C2 to be supplied to the second cylinder group 1b is undesirably decreased toward the lean (thin) side due to the inverse phase control. That is, the air-fuel ratio of the second cylinder group 1b is controlled to be further thinned though the air-fuel ratio is thin.
If the state shown in FIG. 11 has been realized, the air-fuel ratio of the second cylinder group 1b is excessively deviated from the aimed air-fuel ratio. What is worse, the air-fuel ratios of the two cylinder groups 1a and 1b are considerably varied. As a result, deterioration in the purifying efficiency realized by the ternary catalysts 16a and 16b worsens the exhaust gas and changes the engine speed.
As described above, the conventional air-fuel ratio control apparatus for an internal combustion engine has been arranged in such a manner that the air-fuel ratio of the second cylinder group 1b is not feedback-controlled because the exhaust pipe 14b has no air-fuel ratio sensor but it is open-loop-controlled in response to the air-fuel ratio signal AF supplied from the air-fuel ratio sensor 15 disposed in the exhaust pipe 14a for the first cylinder group 1a.
Therefore, if the characteristics of the air-fuel ratio of the cylinder group 1a and that of the cylinder group 1b are considerably different from each other due to the irregular distribution of sucked air caused from the machining deviations between the two cylinder groups 1a and 1b, the structure and layout of the suction pipes, due to the difference in the temperature of sucked air or the engine temperature between the same due to the layout of the cooling water passage and the exhaust pipes 14a and 14b or due to difference in the operation conditions, there rises a problem in that a state is sometimes realized wherein the rich or lean air-fuel ratio is continued.
A state is sometimes continued wherein both of the fuel air ratios of the two cylinder groups 1a and 1b are rich (or lean) at the time of accelerating or decelerating the engine speed. The fact, that the air-fuel ratio of the second cylinder group 1b is controlled in the inverse direction to the direction in which the air-fuel ratio of the first cylinder group 1a is controlled, causes the air-fuel ratio of the second cylinder group 1b to be made further rich though the air-fuel ratio has been made rich or that to be made further lean though the air-fuel ratio has been made lean. As a result, the air-fuel ratio of the second cylinder group 1b is considerably deviated from an aimed air-fuel ratio and the air-fuel ratios of the two cylinder groups 1a and 1b are made to be considerably different from each other. Therefore, a problem arises in that the deterioration in the purifying efficiency of the ternary catalysts 16a and 16b worsens the characteristics of the exhaust gas and the rotational speed is changed.